1. Field of the Invention
This invention relates to novel polymeric thickeners and the process of their use. The thickeners of this invention show special utility when used to thicken latex coating compositions, especially coating compositions containing vesiculated polymeric beads as part of the pigmentation.
2. Description of the Prior Art
Water based latex coatings have gained wide acceptance due to their ease of application, hiding power, drying characteristics, durability and the ease of cleaning brushes, rollers, etc. with soap and water. In order to obtain the proper viscosity to maintain the suspension of pigments in the paint and to provide improved application characteristics, these water based paints normally require the addition of a thickener to adjust their viscosity characteristics.
Numerous thickener compositions have been taught in the prior art. Water-soluble materials employed as thickeners include natural gums and resins, such as starch, gum arabic, modified starch products, dextrins, sodium alginates, and others. Synthetic materials employed as thickeners include polyvinyl alcohol, polyacrylamide, polyacrylic acids and salts thereof, methylcellulose and other cellulose derivatives such as carboxy-methylcellulose and hydroxyethylcellulose, polyvinylpyrrolidone, polyvinylmethylether, polyethyleneoxides and copolymers of polyvinylmethylether and maleic anhydride. Other synthetic polymeric thickeners include esters prepared by the esterification reaction between the acid anhydride groups of a copolymer of an alkylvinylether and maleic anhydride with the terminal hydroxyl groups of a nonionic surfactant of a alkylphenoxyethoxyethanol polymer. The resulting water-soluble polymer is a partial ester which contains free acid groups suitable for subsequent neutralization with a basic material, such as an amine or a metal oxide or hydroxide.
None of the prior art thickeners provides a completely acceptable combination of low shear and high shear viscosities. This is especially true in latex paints containing vesiculated beads as part of the pigmentation. Due to the increased hiding power of these beads, and their lower oil absorption compared to typical pigments, water based paints containing these beads can be manufactured at lower solids than corresponding water based paints containing conventional pigmentation, and it is especially difficult to obtain the required film build upon brush or roller application of these coatings.
The high shear rate (e.g. greater than about 5,000 sec..sup.-1) viscosity of a coating primarily effects the application characteristics of film build and brush drag and increased high shear viscosities provide greater film builds. The mediumshear rate (approximately 50 to 1,000 sec..sup.-1) viscosity primarily affects brush load and dripping. Low shear rate viscosity (e.g. less than about 50 sec..sup.-1) primarily affects flow, leveling and sag resistance. With the prior art thickeners, it is extremely difficult to obtain a high shear rate viscosity sufficient to obtain proper film build, especially in lower solids paints, e.g. less than 45% NVM, and still maintain an acceptable low shear viscosity which is necessary for flow and leveling.
The novel thickeners of this invention overcome the limitations of the prior art and provide a thickener which will impart excellent high shear rate viscosities and still allow excellent flow and leveling as the coating dries.